SOHO CDS and SUMER observations of quiescent filaments and their interpretation

¹ Department of Applied Mathematics and Theoretical Physics, University of Cambridge, UK
² Department of Astronomy and Space Science, University of Florence, Italy
³ Institut d'Astrophysique Spatiale, Université de Paris Sud, 91405 Orsay Cedex, France

Corresponding author: G. Del Zanna, G.Del-Zanna@damtp.cam.ac.uk

Received: 2 September 2003
Accepted: 2 February 2004

Abstract

Three quiescent filaments located at different positions on the solar disk were selected from the SOHO CDS data archive: one of them was also observed by SUMER in the raster mode. We investigate the filament-corona transition region (PCTR) emission, to determine whether it is indeed negligible, as found in one previously-analysed case. The observations are interpreted on the basis of two different models: an isothermal (cool) prominence located above the quiet sun transition region (TR) with a portion of the corona below it, and a model composed of several cool threads embedded in the hot coronal plasma without any quiet sun TR below it. The first model indicates that, for all filaments, the PCTR emission at the top of the filament is indeed negligible, and that the chromosphere-corona TR emission under the filament is lower than the average. All filaments have similar column densities, ranging from ~2 to  cm^-2 according to model A, and from 5 to  cm^-2 according to Model B. It is not possible to determine which model better accounts for the observations, on the basis of the two prominences observed above and below the Lyman continuum limit. Model B predicts absorptions that are generally less consistent with the observations, and produces higher column densities. The comparison between the line intensities observed above and below the He I ionization limit provides an estimate of the relative neutral helium abundance N(He I)/N(H I) in the prominences.